The flame retardant performance of polyesters or nylons can be achieved by the incorporation of various types of additives. Typically, halogenated compounds, more specifically, aromatic polybrominated compounds, have been used as flame retardant additives in such polymers. It is generally accepted that these products inhibit radical gas phase reactions from occurring in the flame when these products are ignited. This makes halogenated flame retardants very commonly used additives for different types of polymeric materials including polyesters and nylons. However, during the last fifteen years or so, halogenated flame retardants have come under scrutiny because of ecological concerns. At this time, the flame retardant industry is under pressure to change to flame retardant additives that are perceived to be more environmentally friendly.
Phosphorus-containing products are logical substitutes for such halogenated flame retardants. In some applications, phosphorus-containing additives show as high an activity as the halogenated ones, but the phosphorus-containing additives are less commonly employed. Most of the phosphorus-containing flame retardants provide flame retardant activity through a combination of condensed phase reactions, polymer carbonization promotion, and char formation. These processes obviously depend on the polymer in which such additive(s) are employed. Therefore, specific phosphorus-containing structures need to be designed for various polymers types. Phosphorus-containing flame retardants also provide flame retardant activity through a gas-phase mechanism. However, because phosphorus-containing compounds tend to react with the decomposing polymer in the course of combustion instead of merely being volatilized, high gas-phase activity of phosphorus additives is relatively rare.
In late 1970s and early 1980s various, salts, such as zirconium or zinc salts, of diaryiphosphinates, alkyl-arylphosphinates or dialkylphosphinates were prepared, as illustrated, for example, by U.S. Pat. Nos. 4,180,495; 4,208,321; and 4,208,322. These phosphinate salts were added to PET or copolymerized with the polyester. At levels of 10-20 wt. %, an improvement of flammability retardation, as measured by the oxygen index (OI) of from 1 to 4 units, was observed.
Later on, a variety of alkylphosphinic acid metal salts of zinc (M=Zn) or aluminum (M=Al), as described by formulae I to IV shown below, were tested in PBT (see European Patent Publication No. 794,220). It was found that the aluminum salt of ethylmethylphosphinic acid (I) gave a V-0 rating in the UL-94 test at 15 wt. % loading in plain PBT and at 20 wt. % loading in glass-filled PBT. The calcium salts of dialkylphosphinic acids (M=Ca) were proven to be as efficient as the aluminum salts and provided a V-0 rating in the glass-filled PBT at 20 wt. % loading (see U.S. Pat. No. 5,780,534 and European Patent Publication No. 941,996). These phosphinic acid salts are not particularly efficient in the glass-filled nylons and provide a V-0 rating only at 30 wt. % loading.

European Patent Publication No. 794,191 discloses the use of cyclic aluminum salts of 1-hydroxydihydrophosphole oxide and 1-hydroxyphospholane oxides (see formulae V, VI and VII below) in the non-glass filled PBT and nylons. A V-0 rating in PBT was achieved at 20 wt. %.

The aluminum salts of 1-(methoxyethyl)methylphosphinic (formula VIII below), of (1-ethoxyethyl)methylphosphinic (formula IX below) and of the 1-(methoxyethyl)ethylphosphinic acids (formulae X below) were disclosed in European Patent Publication No. 971,936. These products showed only a V-1 rating at 20 wt. % loading in the glass-filled PBT.

In spite of a relatively high phosphorus content, the aluminum salt of hydroxymethylmethylphosphinic acid (formula XI below) was less efficient and showed a V-2 rating in glass-filled PBT as exemplified in the U.S. Pat. No. 6,303,674.

It has also been recognized that although the aluminum salt of ethylmethylphosphinic acid (Formula I above) and the aluminum salt of diethylphosphinic acid (Formula XII above) are not particularly effective in nylons, they are synergistic with nitrogen-containing products like melamine cyanurate (see U.S. Pat. Nos. 6,255,371; 6,365,071; and 6,503,969) or melamine phosphate (see U.S. Pat. No. 6,207,736). In other words, these combinations are more effective in nylons than are the individual components.
More recently, it was discovered that the flame retardant efficiency of metal salts of alkylphosphinic acids depends, to great extent, on the volatility (tendency to evaporate or sublime) of these salts. For example, it was unexpectedly found (see Provisional U.S. Patent Application Ser. No. 60/581,832 filed Jun. 22, 2004 and PCT Application No. PCT/US05/21726 filed Jun. 20, 2005, the entire contents of which are incorporated by reference herein, that the aluminum salt of diisobutylphosphinic acid (Formula XIII below) is more efficient; especially in nylons, than, for example, the aluminum salt of diethylphosphinic acid (Formula XII above) in spite of the fact that the aluminum salt of diisobutylphosphinic acid contains less percent, by weight, of the flame retardant element, phosphorus, than does the aluminum salt of diethylphosphinic acid. In addition, although the aluminum salt of diisobutylphosphinic acid (Formula XIII) has a higher molecular weight than diethylphosphinic acid (Formula XII), the diisobutylphosphinic acid (Formula XIII) exhibits better volatility than the aluminum salt of diethylphosphinic acid (Formula XII).

As mentioned above, the mechanism for flame retardant activity for phosphorus-containing flame retardants is usually the condensed phase. Phosphorus-containing flame retardants possessing the gas phase characteristic are rare since not only must the flame retardant not react with the decomposing polymer but the phosphorus-containing compound must also possess the right degree of volatility so as not to be lost during processing of the polymer compositions in which they are added (i.e., not volatilize at too low a temperature) and not volatilize at too high a temperature, so as to be inefficient during combustion. Phosphorus-containing flame retardant additives possessing these desired properties are therefore highly desirable.